1. Field of the Invention
The present invention relates to a process for preparing nanoscale titanium dioxide particles coated with an oxide, hydroxide or oxide hydroxide of Al, Ce, Zr and/or Si, or a suspension thereof, and also to titanium dioxide particles obtainable therefrom which have an average particle diameter of below 50 nm.
2. Discussion of Background Information
Titanium dioxide powders are used on the industrial scale, for example as pigments or as UV absorbers. In order to be able to use titanium dioxide also in applications in which transparency is required, for example as a UV absorber in transparent layers, very fine particles are necessary. The preparation of very small particles having dimensions in the nanometer range is known. For instance, DE-A-198 59 852 describes a process for preparing a suspension of rutile particles whose number-average particle diameter is below 20 nm.
Titanium dioxide also has the property of being photocatalytically active. It can catalyze, for example, decomposition reactions under the action of light. Therefore, titanium dioxide particles generally cannot be embedded into a transparent organic matrix, since the matrix is destroyed by the photocatalytical effect under the action of light.
To prevent or reduce this effect, titanium dioxide particles may be coated or enrobed. The organic matrix may thus provide shielding from the processes responsible for the catalytic action. So that the coated titanium dioxide retains the desired functions such as UV absorption, the coating should be transparent for the appropriate light. In general, oxides of silicon, aluminum, zirconium or cerium are applied as the coating.
For example, D. N. Furlong, K. S. W. Sing and G. D. Parfitt in J. Coll. Interf. Sci, 1979, 69, 409 describe a coating process in which an acidic rutile suspension is admixed with sodium silicate and the suspension is then acidified, stirred and filtered. Coated particles of from 200 to 500 nm are obtained. According to R. E. Day, T. A. Eggerton, Coll. and Surf., 23, 1987, 137, coating is effected by adding aluminum sulfate to an acidic suspension of rutile particles, neutralizing with a base such as ammonia or sodium hydroxide solution, filtering and drying.
However, all known processes for particles in the nanoscale range lead only to irregular coatings having nonuniform thickness. When there are coating sites with too low a thickness or uncovered sites, screening and thus prevention or reduction of the photocatalytic effect is no longer ensured. On the other hand, when a larger amount of coating material is used, this results in agglomeration or aggregation of the particles. For example, aggregates of particles are formed which are joined together by the coating composition. Coated titanium dioxide particles having dimensions which are required in transparent applications cannot be prepared according to the prior art, since coated titanium dioxide particles according to the prior art have an average particle diameter of above 150 nm.
It is therefore an object of the present invention to provide a process for coating nanoscale titanium dioxide particles by which coated titanium dioxide particles having a number-average particle size of below 50 nm and less may obtain effective protection with respect to photocatalysis, in order to be able to use titanium dioxide particles for transparent applications in an organic matrix. For this purpose, uniform coating is required.